In recent developments, many materials have been encapsulated with polymers, especially polyureas. This development is extremely significant from the standpoint of safety and handling of otherwise toxic materials. Often the encapsulated materials have provided superior results in the applications for which they are intended. A convenient method of application of the encapsulated materials is by spraying water dispersions. It has been found that the suspensions of encapsulated materials are not stable and tend to separate rapidly, even when conventional dispersants are used, thereby resulting in caking of the microencapsulated material.
The application of suspensions which have separated is extremely difficult under field conditions and, therefore, stable suspensions are desirable. Polyurea and other encapsulated materials on separating and standing, cake excessively, such that the particles cannot be redispersed with equipment normally used in field applications. This produces an unsatisfactory condition for actual field use.
Reference is made to Belgian Pat. No. 796,746 assigned to Stauffer Chemical Company, published Sept. 14, 1973. The aforementioned patent describes a method for encapsulating various water-immiscible materials employing an organic isocyanate intermediate to form a polyurea capsule enclosure around a water-immiscible material dispersed in an aqueous continuous phase. Capsules of this nature and description have a variety of uses, such as for containing dyes, inks, chemical reagents, pharmaceuticals flavoring materials, fungicides, bactericides, pesticides, such as herbicides, insecticides, and the like, which substances can be dissolved, suspended or otherwise dispersed in or as the material to be enclosed by the polyurea capsule. The material to be encapsulated can be employed in the initial dispersion if a liquid, or heated to a temperature above its melting point if a solid, or dissolved or dispersed in suitable water-immiscible organic solvents. Once encapsulated, the liquid or other form is preserved until it is released by some means or instrumentality that breaks, crushes, melts, dissolves, or otherwise removes the capsular skin or until released by diffusion is effected under suitable conditions. Effective encapsulation by interfacial polymerization of an organic isocyanate intermediate can be accomplished in a process which utilizes two substantially immiscible liquids, one termed an aqueous phase and the other termed an organic phase, which comprises establishing a physical dispersion of the organic phase in the aqueous phase. Said organic phase contains the isocyanate intermediate for the polyurea capsule skin or enclosure. The interfacial polymerization, as it is termed, involves hydrolysis of an isocyanate monomer to form an amine, which in turn reacts with another isocyanate monomer to form the polyurea enclosure. During the hydrolysis of the isocyanate monomer, carbon dioxide is liberated. Capsules formed in this matter can range from 0.5 microns to about 100 microns.